Stepenovanje ogromnim brojem

#include <bits/stdc++.h>
typedef long long ll;
using namespace std;

const ll mod=1e9+7;
const int N=1e5+10;

ll mod_pow (ll num, ll pw)
{
    ll ret=1;
    while (pw)
    {
        if (pw&1) ret=(ret*num)%mod;
        pw>>=1;
        num=(num*num)%mod;
    }
    return ret;
}

ll a, b, sol=1;
char s[N];

void calc ()
{
    int n=strlen(s);
    ll x=a;

    for (int i=n-1;i>=0;i--)
    {
        sol=(sol*mod_pow(x, (ll)(s[i]-'0')))%mod;
        x=mod_pow(x, 10);
        b/=10;
    }
}

int main()
{
    scanf ("%lld%s", &a, s);

    calc();
    cout << sol;

    return 0;
}